1. Field of the Invention
This invention relates to an exposure time reproducing device which digitally reproduces an exposure time and more particularly to a digital exposure time reproducing device which reproduces a digital datum relative to a logarithmically suppressed exposure time into an actual exposure time.
2. Description of the Prior Art
In the conventionally known exposure reproducing device of the above stated kind, one of several read-out pulse trains is selected according to the information in the integral part of an exposure time datum (information on an exposure time value nEV, wherein n represents an integer); the number of pulses of the selected pulse train is counted concurrently with opening of a shutter; and the shutter is arranged to be closed when the counted value thus obtained coincides with a value corresponding to information in the decimal part of an exposure time datum (information on an exposure time of 1/n EV, wherein n represents an integer). However, such an exposure time reproducing device necessitates as many read-out pulses as the number of stepped values of exposure time to be controlled, i.e. the number of controlled exposure time values EV. Where the exposure time to be controlled ranges from 1/2000 sec to 1 sec, the number of the exposure time values, for example, is 11. Therefore, a circuit arrangement for producing the read-out pulse trains becomes more complex as the exposure time value reproducing range becomes wider. Because of this problem, the conventional exposure time reproducing devices have not been suited for high-grade cameras requiring exposure control over a wide range.
Another shortcoming of the conventional devices of this kind has resided in that the functions of the systems as a whole become inflexible with one circuit arranged for each function independently of another. This inflexibility has been defying all attempts to make additions and/or modifications of the device.
To solve the problems of these conventional devices, a device was proposed by Japanese laid-open patent application No. 55-105232, published Aug. 12, 1980. The proposed device uses a micro-computer. In accordance with this prior art device, a logarithmically suppressed digital exposure time datum is expanded by shifting the datum of the decimal part thereof according to the contents of the integral part thereof. Then, the expanded datum is read out by means of pulses of a predetermined frequency.
However, this prior art device is hardly practicable for lack of consideration of exposure errors that result, in a camera using a slit shutter, from the overlapping of the leading and trailing curtains of the shutter and a response delay time of a magnet provided for the purpose of controlling the trailing curtain.